1. Field of the Invention
The present invention relates generally to radio systems and, more particularly, to filters with a wide frequency tuning range.
2. Description of Art
Bandpass filters are widely used in electronic communication systems. In conventional radio transceivers with a single standard and fixed band of operation, bandpass filters are designed for a predetermined center frequency and bandwidth. Fixed RF filters are usually realized using lumped element L-C resonators, transmission-line and waveguide resonators, ceramic resonators, surface acoustic wave (SAW) resonators, and most recently, film bulk acoustic resonators (FBAR).
In multi-standard single-platform radio applications, such as software-defined radio (SDR) transceivers, however, the frequency range of interest spans nearly two decades. A typical SDR transceiver may cover from approximately 30 and 3000 MHz. This wide frequency range is typically divided into a number of narrower bands. Radio frequency (RF) bandpass filters in the existing SDR transceivers are realized in the form of switchable filter banks. An N-band switchable filter bank is composed of two single-pole-N-throw (SPNT) selector switches and an array of N-fixed-frequency bandpass filters. At a given mode of operation, depending on the frequency of interest, the SPNT switches route the RF signal through only one of the filters, and bypass the rest. As the number of bands is generally large for wideband SDR, the filter bank approach drastically increases the size, weight, and manufacturing cost of the SDR RF front-end. Also, as the filters for different bands are generally implemented using different technologies, integrating a large number of these filters in a single module can be very challenging.
In an alternative scenario, RF filtering can be achieved in a more compact fashion, by using programmable MEMS switched-band filters. Requirements for such filters are often low insertion loss, high linearity, maximum power handling, and minimum complexity. Competing technologies for tunable filter designs are based on using solid-state varactors, p-i-n diode switches, MEMS varactors, and ferroelectric varactors. Varactor-based designs typically suffer from insufficient tuning range and are mostly power inefficient. Switch-based designs using solid state devices typically offer poor performance in terms of insertion loss and linearity.
MEMS-based tunable and switched-band filters reported in the existing literature typically have tuning ranges of less than one octave and narrow bandwidth. MEMS tunable filters have been described by Entasari et al., in “A 12-18-GHz Three-Pole RF MEMS Tunable Filter”, IEEE Transactions on Microwave Theory and Techniques, Volume 53, Issue 8, August 2005 Page(s): 2566-2571, by Pillans et al., in “6-15 GHz RF MEMS tunable filters”, 2005 IEEE MTT-S International, 12-17 Jun. 2005 Page(s): 919-922, by Entasari et al. in “A differential 4-bit 6.5-10-GHz RF MEMS tunable filter”, IEEE Transactions on Microwave Theory and Techniques, Volume 53, Issue 3, Part 2, March 2005 Page(s): 1103-1110, by Abbaspour-Tamijani et al. in “Miniature and tunable filters using MEMS capacitors”, IEEE Transactions on Microwave Theory and Techniques, Volume 51, Issue 7, July 2003 Page(s): 1878-1885, by Lee et al., in “Millimeter-wave MEMS tunable low pass filter with reconfigurable series inductors and capacitive shunt switches”, IEEE Microwave and Wireless Components Letters [see also IEEE Microwave and Guided Wave Letters], Volume 15, Issue 10, October 2005 Page(s): 691-693, by Fourn et al., in “MEMS switchable interdigital coplanar filter”, IEEE Transactions on Microwave Theory and Techniques, Volume 51, Issue 1, Part 2, January 2003 Page(s): 320-324, and by Carey-Smith et al. in “Wide tuning-range planar filters using lumped-distributed coupled resonators”, IEEE Transactions on Microwave Theory and Techniques, Volume 53, Issue 2, February 2005 Page(s): 777-785, all of which are incorporated herein by reference.
MEMS tunable filters with broader tunable ranges have also been described by Allison et al., in U.S. Pat. No. 6,784,766, incorporated herein by reference.